1. Field of the Invention
The present invention relates to a manufacturing method of a resin for developing agent which is used in image forming devices, for example, copiers and printers.
2. Description of the Related Art
In general, in an image forming device, an electrical latent image is first formed on an electrostatic latent image carrier such as a photoreceptor. This latent image is developed with a toner. The developed toner image is transferred onto a transfer material such as paper. Then, an image is formed through fixation by heating or pressurization. A toner particle which is used for the image formation is mixed with a carrier particle and used as a two-component system developing agent. Alternatively, a magnetic toner particle or a non-magnetic toner particle is used as a single-component system developing agent in a single body.
In general, a toner particle is constructed of materials including a resin which becomes a binder, a coloring agent, a mold releasing agent such as waxes, and a charge inhibitor. In recent years, an emulsion polymerization coagulation method is employed as a formation method of toner particle.
The emulsion polymerization coagulation method is to form a toner particle by the following process. First of all, a dispersion of a resin as formed by emulsion polymerization is prepared, whereas a coloring agent dispersion having a coloring agent dispersed in a solvent is prepared. In addition, a dispersion of a wax is prepared, if desired. Then, by mixing the resin dispersion and the coloring agent dispersion and optionally the dispersion of a wax, a coagulated particle corresponding to the toner particle diameter is formed. Then, for example, by fusing the coagulated particle by heating, a fused particle is obtained, followed by washing, filtration and drying to form a toner particle.
According to this emulsion polymerization coagulation method, by selecting conditions such as heating temperature, it becomes possible to control the shape or surface composition of the toner particle in a prescribed form of, for example, from an amorphous form to a spherical form. By stably controlling the particle shape of the toner in a prescribed particle size, it is possible to suppress a lowering of developability and deterioration of image quality. Furthermore, by controlling the surface composition of the toner particle such that the wax is not exposed on the surface of the toner particle, it is possible to suppress contamination due to separation of the wax. In addition, by controlling the surface composition of the toner particle such that the coloring agent is not exposed on the surface of the toner particle, it is possible to suppress defective electrification. Accordingly, improvements in image quality and reliability can be expected.
In the thus formed toner particle, an improvement in fixability to a transfer material is required. Due to the improvement in fixability, it becomes possible to achieve more improvements in image quality and reliability. For improving the fixability, it is required to fix the toner particle to a transfer material with low energy and to suppress the generation of an offset phenomenon at high temperatures. In general, in the resin which becomes a binder in the toner particle, one having a low molecular weight is softened at low temperature so that it can be fixed to a transfer material with low energy. However, due to a lowering of viscoelasticity at high temperatures, an offset phenomenon is generated. On the other hand, one having a high molecular weight suppresses a lowering of viscoelasticity at high temperatures. However, its softening temperature is high so that high energy is required for fixation to a transfer material. In this way, a characteristic to control the fixability varies depending upon the molecular weight of the resin. Accordingly, in order to improve the fixability, it is necessary to control the molecular weight distribution of the resin in forming a toner particle.
Usually, in order to control the molecular weight distribution, the molecular weight must be adjusted in a stage of polymerizing a resin from a monomer. Then, for example, as described in JP-A-2003-231707, there is proposed a measure in which a chain transfer agent is added at a point of time when a degree of polymerization is from 10 to 80%, thereby forming a resin particle for toner. However, a measure capable of more easily controlling the molecular weight of a resin particle is required.